Iocco, F

Serpico, Pasquale Dario

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PoS(NIC-IX)201Synthesis of CNO elements in standard BBNFabio Iocco∗Università di Napoli "Federico II"Dip. Scienze Fisiche, via Cintia, 80126 Napoli, ItalyKavli Institute for Particle Astrophysics and CosmologyPO Box 20450, MS 29, Stanford, CA 94309, USAE-mail: iocco@na.infn.itPasquale D. SerpicoMax Planck Institut für Physik, (Werner Heisenberg Institut),Föhringer Ring 6, 80805, München, GermanyE-mail: serpico@mppmu.mpg.deWe perform an analysis of the CNO elements production in a standard big bang nucleosynthesis(BBN) scenario. The goal is to provide a sound estimate on the very low but yet poorly exploredabundance of such elements (with critical importance in Population III Stars). We examine thesynthesis channels for these elements in the Wagoner-Kawano code (already critically revisedand updated in recent analysis); in light of the findings on CNO elements productions in BBN weadd 4 nuclides and more than 200 reactions to the existing network; our results show no majordiscrepancies with the ones from smaller nuclear network. The robustness of the standard BBNpredictions—Population III Stars were metal-free—is confirmed.International Symposium on Nuclear Astrophysics — Nuclei in the Cosmos IXJune 25-30 2006 CERN, Geneva, Switzerland∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC-IX)201Synthesis of CNO elements in standard BBN Fabio Iocco1. Primordial Nucleosynthesis and Population III starsStandard BBN is a one-parameter theory, mostly depending on well known nuclear and particlephysics processes at the keV-MeV scale. The only unknown parameter is the baryon fraction ωb,which actually depends on physics beyond the standard model. Nonetheless, nowadays ωb canbe determined independently via cosmic microwave background (CMB) anisotropy studies [1].Using the CMB value for ωb turns BBN into a parameter-free (and thus highly predictive) theory,which can be used to check the internal consistency of the Standard Cosmological Model (SCM),to constrain astrophysics (e.g. 7Li and 3He stellar reprocessing) or for the exploration of exoticphysics, as in [2]. In recent analyses like [3] the nuclear channels for the synthesis of light elementsup to mass number A = 7 have been carefully studied. The relevant reaction rates have also beenupdated with the newest experimental and theoretical estimates, thus reducing the uncertainties onthe light element abundances. Indeed, the main uncertainty on nuclear abundances arises fromthose on nuclear rates; however this is true only when the reactions relevant for the synthesis of theelements of interest are included. The widely used Wagoner-Kawano code contains nuclides up to16O, but no detailed analysis of the completness of the network for mass number A> 7 is presentin the literature. Missing reactions might have huge effects on the synthesis of CNO elements,although being of negligible entity for the “traditional” BBN predictions of light elements yields.Studies on the stars formed from the collapse of the very first structures in the Universe haveshown that the physics of such objects is strongly dependent from the chemical composition of thecloud [4, 5]. Namely, even relatively small amounts of Carbon, Nitrogen and Oxygen could modifyeither the collapse of the cloud and/or the evolution of the smallest among these stars [6]. The verylow fraction of CNO elements necessary to modify the physics of these processes (respectively10−4 and 10−11) has motivated the current study; our purpose is to explore the synthesis channels ofCarbon, Nitrogen and Oxygen in BBN and put sound constraints on their abundances by checkingthe consistency of the nuclear network and correctly estimating uncertainties.2. Current nuclear network analysisWe have analized the synthesis channels for Carbon, Nitrogen and Oxygen in the currentlyused network in BBN, as critically modified for the light elements network in [3]. We have foundthat the small amounts of Carbon produced are synthesized via the direct reaction channel7Li(α ,γ)11B(p,γ)12C, (2.1)or the secondary one7Li(α ,γ)11B(d,n)12C. (2.2)No one of these reactions is responsible for the final abundance of 11B; during BBN they do infact compete with the stronger 11B(p,α)2 4He, which depletes all the 11B. The final abundance ofthis element is provided by the β decay of 11C produced by α capture on 7Be, which is Coulomb-barrier suppressed with respect to the analogous α capture on7Li. The 11B left at the end of the2PoS(NIC-IX)201Synthesis of CNO elements in standard BBN Fabio IoccoFigure 1: 12C(×10) , 14C(×102) , 14N(×102) , 16O(×105) abundances as a function of the baryon densityωb within the 1σ interval ωb = 0.0223± 0.0008 as from [1]. The 4He mass fraction (×10−14) is reportedas well to show how they have the same dependence from this parameter; this is a direct consequence of theCNO elements production taking place mainly through the single channel 7Li(α ,γ)11B.BBN is therefore produced through the channel7Be(α ,γ)11C→ 11B (2.3)Another possible path is via the unstable nuclei with A= 8,8Li(α ,γ)12B → 12C (2.4)8B(α ,γ)12N → 12C, (2.5)where 8Li is produced e.g. via 7Li(n,γ) and 6Li(t, p), and 8B via 7Be(p,γ). According to ourestimates, however, these channels only provide a sub-leading contribution to the CNO abundances.The production of heavier isotopes of Carbon, Nitrogen and Oxygen proceeds by radiativeproton capture on carbon nuclei, followed by a (n, p) reaction. For example,13C is produced alongthe following channel from light elements7Li(α ,γ)11B(p,γ)12C(p,γ)13N(n, p)13C (2.6)and 14C proceeds along from 13C according to the same path13C(p,γ)14N(n, p)14C. (2.7)3PoS(NIC-IX)201Synthesis of CNO elements in standard BBN Fabio IoccoNitrogen is produced by means of proton radiative capture on Carbon nuclei—e.g.12C(p,γ)13N—and heavier isotopes like Oxygen ones are produced with the same mechanism illustrated so far.Note that once heavier elements (A ≥ 12) have been synthesized, they are not disrupted back intolight elements. It is worth noticing how the mechanism for 12C production is completely differentfrom the well known 3 α process taking place in stars. This is easily explained in terms of (i)the very low density of the plasma during BBN (less than 1020 baryons/cm3 at T <∼ 100 keV); (ii)the Coulomb suppression relevant for heavier nuclei; (iii) the short time-scales involved, of theorder of minutes. Synthesis of CNO elements in BBN does hence proceed along a path involvingintermediate mass nuclides (4 < A < 12); it is therefore conceivable that relevant channels mayhave been omitted when neglecting intermediate mass elements.3. Updating the BBN codeWe have modified the code presented and used in [3] adding 4 nuclides and more than 200nuclear reactions previously neglected. The criterion adopted for adding new nuclides is based ontheir half-lives t1/2 ’s; we have found that 4 nuclides previously neglected in the Kawano code havet1/2 ’s comparable or longer than the typical times involved in BBN. We have therefore included9Li (t1/2=0.178 s),10Be (t1/2=1.5×106 years), 9C (t1/2=0.125 s), 10C (t1/2=19.29 s). By addingthe main reactions involving these nuclides we are including all viable "intermediate" channelconnecting lighter to heavier elements. We have added reactions for these nuclides mimickingthe structure of the pre-existing nuclear network, and updating where necessary. The reactionsincluded for these nuclides (and the ones added for the nuclides already present in the code) havebeen selected on basic physical arguments, first of all Coulomb barrier considerations: Reactionswith Z1Z2 ≥ 12 have not been considered as they are mostly suppressed in the late times of BBN,when metals are mainly formed, and energy is very low, namely few keV. For each nuclide a fullnetwork including radiative capture, stripping/pick-up and charge-exchange up to3He has beenimplemented. For example, a list of these reactions for the neutron case is shown below• radiative capture: (n,γ); (p,γ); (d,γ)• neutron stripping: (n,d); (n,3He)• 2H pickup: (d, p); (d,n)• charge exchange: (p,n); (p,n); (3He, t); (t,3He).Many of these reactions were missing in the original nuclear network of the Wagoner-Kawanocode; in fact they do not have appreciable effects on the light nuclide abundances, which was theprincipal goal of the original codes. To evaluate the missing reaction rates, extensive use has beenmade of complilations and on-line libraries like [7, 8, 9]. The nuclear rates for many of thesereactions, where measurements or theoretical estimates were missing, have been estimated withsimple nuclear models, of the kind presented in [10]. An order of magnitude uncertainty has beenassumed. After adding these reactions we have re-analized the synthesis channels for C, N, and O.We can summarize the main features of our analysis as follows:4PoS(NIC-IX)201Synthesis of CNO elements in standard BBN Fabio IoccoNuclide 9Be/H 11B/H 12C/H 13C/H 14C/H 14N/H 16O/H(×10−19) (×10−16) (×10−16) (×10−17) (×10−17) (×10−17) (×10−20)Original 2.4 3.9 4.4 7.6 0.6 2.6 1.8Updated 2.5 3.9 4.6 9.0 1.3 3.7 2.7Table 1: Some “heavy” nuclei abundances (for ω b=0.023) predicted respectively by the code in [3] and byusing the upper limits to the production rates in the new version described in the text.• identification of relevant channels for CNO synthesis in the existing code• update of the rates of relevant reactions already present• addition of 4 nuclides: 9Li, 10B, 9C, 10C• addition of more than 200 nuclear reactions previously neglectedAs it can be seen from Tab. 1, no major change is found when enlarging the network despite usingupper limits for the rates in the new updated version of the code. The physical reason is that at thebeginning of the BBN, when the higher temperature and density would favor the formation of CNOelements, their synthesis is suppressed by the high entropy and the lack of necessary intermediateelements. On the other hand, when enough “catalyzers” are produced at late times temperatureand density are not high enough to favor the reactions by overcoming the high Coulomb barrier.Of course, all the newly introduced nuclei take part to the nucleosynthesis, but they decay or areprocessed into heavier ones by the end of BBN.Even taking into account a much enlarged network, one might still wonder how robust areprevious estimates, especially given the lack of data or detailed theoretical models for several re-actions. To obtain a generous but more robust upper limit to the abundance of Carbon, Nitrogen,and Oxygen in BBN, we have performed an analysis of the final abundances of these elementsassuming that all the reactions producing at some stage an element with A > 7 “instantaneously”produce 12C. Namely, the yield of the A> 7-product is formally considered as a CNO nucleus. Inthis way, we neglect any destruction mechanism (since CNO elements are more stable, they act asa monothonically growing reservoir), the multistep nature of the synthesis of CNO nuclides, andmaximize the time available for their synthesis. As a result, we get CNO/H =≤ 4× 10−9. Thisbound only depends on thermodynamical properties of the plasma at the BBN conditions, and onthe well-known nuclear rates involving nuclei with A≤ 7.4. ConclusionsApart for predicting detailed values for the abundances of the light nuclei2H and 3He, 4He,and 7Li, standard BBN also predicts that the first collapsed objects in the Universe should haveno appreciable “metallicity”. This prediction is crucial for determining the physical mechanismsregulating the collapse of the clouds leading to the PopIII stars, and the evolution of the smallestamong these pristine stars.Given the importance of this topic, and the incompleteness of standard BBN nuclear networksin the mass range A>7, we have performed a detailed study of the synthesis channels of Carbon,5PoS(NIC-IX)201Synthesis of CNO elements in standard BBN Fabio IoccoNitrogen and Oxygen in Primordial Nucleosynthesis. We have found that Carbon is produced in away consistently different from the usual 3α reaction in stars. The synthesis of12C proceeds alongpaths involving intermediate mass elements, namely11B and 11C. The very light traces of Nitrogenand Oxygen are produced by radiative capture upon 12C. We have added to the code 4 nuclidesand more than 200 reactions observing no sensible increase of CNO elements. Re-analysing thesynthesis of CNO after the addition of the new reactions we find that the main channels for theirproduction stay the same as before and none of the newly added reactions/nuclides opens effectivechannels from light to heavy elements in BBN. We regard this as a final and sound proof that onlynegligible traces of CNO elements are produced in standard BBN, in agreement with earlier andless accurate analyses. This should be regarded as a further observational test for standard BBN,since alternative theories like Inhomogeneous BBN might lead to significantly different conclu-sions (see e.g. [11] and Refs. therein).References[1] D. N. Spergel et al., “Wilkinson Microwave Anisotropy Probe (WMAP) three year results:Implications for cosmology,” astro-ph/0603449.[2] A. Cuoco, F. Iocco, G. Mangano, G. Miele, O. Pisanti and P. D. Serpico, “Present status of primordialnucleosynthesis after WMAP: results from a new BBN code,” Int. J. Mod. Phys. A 19, 4431 (2004)[astro-ph/0307213].[3] P. D. Serpico, S. Esposito, F. Iocco, G. Mangano, G. Miele and O. Pisanti, “Nuclear ReactionNetwork for Primordial Nucleosynthesis: a detailed analysis of rates, uncertainties and light nucleiyields,” JCAP 0412, 010 (2004) [astro-ph/0408076].[4] F. Nakamura and M. Umemura, “On the Initial Mass Function of PopulationIII Stars,” ApJ 548(2001), 19N [astro-ph/0010464].[5] T. Abel, G. L. Bryan and M. L. Norman, “The Formation of the First Star in the Universe,", Science295, 93 (2002) [astro-ph/0112088].[6] I. Dominguez, O. Straniero, M. Limongi and A. Chieffi, “Do zero metal intermediate mass starsexperience thermal pulses?,” astro-ph/9912425.[7] Audi et al., “The Nubase evaluation of nuclear and decay properties,” Nuc. Phys.A 624 (1997), 1.[8] Table of Isotopes, 8th edition, online at http://www.tunl.duke.edu/nucldata .[9] See the URL http://www.nndc.bnl.gov/exfor3/endf00.htm.[10] R. Wagoner, W. Fowler, F. Hoyle, “On the Synthesis of Elements at Very High Temperatures,” ApJ148 (1967), 3W.[11] K. Jedamzik, “Did the Universe start at Zero Metallicity?,” astro-ph/9911242.6

Synthesis of CNO elements in standard BBN

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